As pointed out in my aforementioned co-pending U.S. application, an extensive art, including U.S. Pat. Nos. 3,726,643; 4,431,448; 4,710,348; 4,751,048; 4,772,452; and 4,774,052, is known which describes the production of metal-second phase composites by self-propagating high-temperature synthesis (SHS); i.e. an exothermic, self-sustaining reaction which propagates through a mixture of compressed powders until all the fuel for the reaction is consumed. Numerous combinations of materials have been utilized including, for example, mixtures of a metal from Groups II, V, and VI of the periodic system with a non-metal such as carbon, boron, silicon, sulfur, liquid nitrogen, etc. Titanium and carbon may be utilized. A hard alloy may be prepared by mixing titanium, boron and carbon with a Group IB binder metal such as copper or silver to produce an alloy comprising titanium diboride, titanium carbide and the binder metal. Ceramic or phase-forming constituents may be reacted in the presence of a solvent metal. Compound starting materials, as opposed to elemental starting materials may be employed. For example, Al B.sub.12, Ti and Al powders may be blended, compacted and reacted to produce a composite comprising TiB.sub.2 particles dispersed in an aluminum matrix. The proportions of ingredients may be widely varied.
In all of the methods and materials employed to date in SHS reactions, a common there is present, i.e. a mixture of reactant powders is compacted and the compact is ignited in a protective atmosphere such as argon.
The typical method for igniting SHS reactions is to contact compacted powders with a hot (typically tungsten) wire. A modification of this procedure comprises use of an induction coil to heat a graphite block which contacts the powder compact. Another method of ignition being used is a small electrical or chemical fuse.
In most cases, these methods work well under research conditions, but do not lend themselves readily to production scale-up. In the case of the tungsten wire, the wire is often destroyed upon ignition of the powders and must be replaced frequently. In addition, it has been observed that the hot wire approach will not always ignite compacts produced from powders with a particle size larger than 325 mesh (44 microns). The heated graphite block approach requires approximately one minute to heat the graphite block and initiate the reaction. Both of these methods could become quite time-consuming in a production mode.
The fuse method is not appropriate for two reasons. First the fuses are a lost raw material cost. Secondly, since the fuse must be buried in the material being reacted, each piece must be handled to insert the fuse. In addition, the fuse area is contaminated. Since many of these reactions are being used to produce near net shape monolithic parts (which are usually quite small) the chemical contamination, fuse costs and installation labor are prohibitive.